In a generally known structure of an organic electroluminescence element (hereinafter referred to as “organic EL element”), an anode made of a transparent electrode, a hole transport layer, a light emitting layer, an electron injection layer, and a cathode are stacked on a surface of a transparent substrate in this order. In this organic EL element, light is produced in an organic light emitting layer in response to application of voltage between the anode and the cathode, and the produced light passes through the transparent electrode and the transparent substrate and emerges outside.
Generally, the organic EL element has the light-outcoupling efficiency in the range of about 20% to 30%. Such low light-outcoupling efficiency means that 70% to 80% of the total amount of produced light does not effectively contribute to light emission. This is because, due to total reflection at interfaces between materials with different refractive indexes, light absorption by materials, and the like, light cannot be effectively propagated to an outside in which light emission is observed. Consequently, it is considered that improvement of the light-outcoupling efficiency causes a great increase in the efficiency of the organic EL element.
There is studied and developed actively to improve the light-outcoupling efficiency. Especially, there have been many efforts to increase the amount of light which is produced in the organic layer and reaches the substrate layer. Generally, the refractive index of the organic layer is equal to or more than about 1.7, and normally the refractive index of the glass layer serving as the substrate is about 1.5. Therefore, a loss caused by total reflection at the interface between the organic layer and the glass layer (thin film waveguide mode) probably reaches about 50% of the total amount of radiated light. In view of this, it is possible to greatly improve the light-outcoupling efficiency of the organic EL element by decreasing the loss caused by the total reflection between the organic layer and the substrate.
Providing one or more diffraction gratings between the organic layer and the substrate is considered as a method for reducing total reflection loss. For example, patent literature 1 (JP 2991183 B2) proposes extracting a larger amount of light by changing angles of rays of light with one or more diffraction gratings.
Further, use of interference is considered as one of means for improving the light-outcoupling efficiency. For example, patent literature 2 (JP 2004-165154 A) discloses adjusting optical thicknesses by use of the interference in view of differences of phases so as to maximize a component of light.